Washing 101
One of the many great things about wool is how infrequently it needs to be washed. Aside from being soiled, generally, wool only needs washing once it starts to smell less than fresh or it starts to feel damp to the touch (if your wool diaper covers have been soiled, you will want to get those rinsed/soaking as soon as possible).
So why is it wool can come into contact with urine over and over and simply be aired out? Wool is naturally rich in lanolin (and through the process of lanolizing, we ensure it remains so). Lanolin is a fatty acid - it has a slightly acidic ph. Urine on the other hand is a weak base having a slightly alkali ph. When urine comes into contact with the lanolin it is neutralized and the end result is water and a form of salt. Because of wool's great breathable feature, the water is released as a vapor leaving behind the salt residue.
Not only must the salt residue be washed away (it can be harmful to your wool), but once all the available lanolin in the wool has reacted with urine, the only thing additional urine has to react with is your nose! Hence, this is why we wash the wool when it smells a little less than fresh. Also, once the lanolin is used up, it won't repel moisture as well.
Washing wool is a pretty simple process once you figure out what works best for you. There are several product options and methods to wash wool and every experienced wool user will have their own unique method to the madness.